Magnesium alloys, being one third lighter than an equal volume of aluminum, are the lightest structural material in the car industry. The vehicle weight and fuel economy are becoming increasingly important in the automotive industry. The European and North American car producers have committed to reduce the fuel consumption by 25% and thereby to achieve 30% reduction of the CO2 emissions by the year 2010. Accordingly, the said alloys are becoming still more attractive.
Most of the drive train components are produced by high-pressure die casting. This technique has probably the greatest production volume among procedures employing magnesium alloys, and it seems to remain so even in future. However, also other techniques are used, including sand casting and permanent mold casting, squeeze casting, semi-solid casting, thixocasting and thixomolding.
The cost of an alloy represents a significant proportion of the total component cost, becoming an important factor in the development of new alloys. An ideal magnesium alloy for making automobile parts, beside being cost effective, should meet several conditions related to its behavior during the casting process and during its use under continued stress. The good castability includes good flow of melted alloy into thin mold sections, low sticking of the melted alloy to the mold, and resistance to oxidation during the casting process. A good alloy should not develop cracks during cooling and solidifying stage of casting. The parts that are cast of the alloy should have high tensile and compressive yield strength, and during their usage they should show a low continued strain under stress at elevated temperature (creep resistance). The good mechanical properties should be kept even at temperatures higher than 120° C., if the parts are intended as parts of the gear box of a crankcase. However, some drive train components, such as engine block, oil pan, intake manifold, lower crankcase, oil pump housing and others, should withstand even higher temperatures. Improved creep resistance and stress relaxation properties are a critical issue for the alloy to be used for manufacturing such components. The alloy should also be resistant to the corrosion. The physical and chemical properties of the alloy depend in a substantial way on the presence of other metallic elements which can form a variety of intermetallic compounds. These intermetalic compounds impede grain sliding under stress at elevated temperatures.
One of procedures known in the art for improving stability of a metallic mixture is a type of heat treatment, called ageing, which can affect the microstructure of the metal. However, the existing commercial die cast magnesium alloys do not exhibit a marked response to ageing.
All conventional die casting magnesium alloys are based on Mg—Al system. The alloys of the Mg—Al—Zn system (e.g., commercially available alloy AZ91D) or of Mg—Al—Mn system have good castability, corrosion resistance and combination of ambient strength and ductility, however they exhibit poor creep resistance and poor elevated-temperature strength. On the other hand, Mg—Al—Si alloys and Mg—Al—RE alloys have better creep resistance but exhibit insufficient corrosion resistance (AS41 and AS21 alloys) and poor castability (AS21 and AE42 alloys). Both types of alloys further exhibit relatively low tensile yield strength at ambient temperatures. In addition, high content of RE elements, e.g. 2.4% in AE42, increases the costs.
The introduction of other alloying elements in the alloy may overcome some of the mentioned drawbacks. German Patent Specification No 847,992 describes magnesium-based alloys, which contain up to 3 wt % calcium, showing a creep strain of less than 0.2% under an applied stress of 30 MPa at 200° C. for 50 hours. GB 2,296,256 discloses a magnesium-based alloy containing up to 2 wt % RE and up to 5.5 wt % Ca, claiming the creep rate of 0.01% per 50 hours. WO 9625529 discloses a magnesium-based alloy containing up to 0.8 wt % calcium which has a creep strain of less than 0.5% under an applied stress of 35 MPa at 150° C. for 200 hours. EP 799901 describes a magnesium-based alloy for semi-solid casting which contains up to 4 wt % calcium and up to 0.15 wt % strontium, wherein the ratio Ca/Al should be less than 0.8. EP 791662 discloses magnesium-based alloy comprising up to 3 wt % Ca and up to 3 wt % of RE elements, wherein the alloys are die-castable only for certain ratios of the elements, claiming enhanced strength at higher temperatures. EP 1048743 teaches a method for making a magnesium alloy for casting, comprising Ca up to 3.3% and Sr up to 0.2%, claiming an improved creep resistance at 150–175° C. WO 01/44529 claims an alloy for die-casting which contains up to 7% strontium, and which has a creep deformation of 0.06% at 150° C. U.S. Pat. No. 6,139,651 discloses a magnesium-based alloy comprising Ca up to 1.2 wt %, Sr up to 0.2 wt %, RE elements up to 1 wt %, beryllium up to 0.0015 wt %, while Zn is in one of the ranges 0.01 to 1 wt %, and 5 to 10 wt %. This alloy exhibits excellent castability, corrosion resistance and mechanical properties, and is designated for applications with operating temperature up to 150° C. However, in order to expand magnesium applications to crankcase and engine blocks operating at temperatures higher than 150° C., still more enhanced resistance of the alloys is required. It is therefore an object of this invention to provide magnesium alloys capable of operating at temperatures as high as 175–200° C. This invention aims at providing alloys with improved strength at ambient and elevated temperatures, as well as improved creep resistance at elevated temperatures up to the temperatures in the range of 175–200° C.
It is another object of this invention to provide alloys, which are particularly well adapted for high pressure die-casting process, exhibiting reduced susceptibility to die sticking, oxidation, and hot cracking, and which have good fluidity.
It is still another object of this invention to provide magnesium-based alloys suitable for elevated temperature applications which have a good corrosion resistance.
It is a further object of this invention to provide alloys, which may also be used for other applications such as, sand casting, permanent mold casting, squeeze casting, semi-solid casting, thixocasting and thixomolding.
It is a still further object of this invention to provide alloys, which can be successfully cast though being beryllium free.
This invention also aims at providing alloys that exhibit improvements of their strength in course of ageing.
It is also an object of this invention to provide alloys, which exhibit the aforesaid behavior and properties and have a relatively low cost.
Other objects and advantages of present invention will appear as description proceeds.